Indian Journal of re & Applied Physics Vol. 37, June 1999, pp. 449-454

Detection of fast burst of neutrons in the background of intense electromagnetic pulse

Anurag Shyam * Neutron Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085

Received 25 January 1999; accepted 5 April 1999

There are many experiments, in which fast neutron burst is emitted along with strong electromagnetic pulse. This pu lse has frequen cy spectrum starti ng from few tens of kHz to hard X-rays. Detecting these neutrons bursts req uire special measurement techniques. which are described.

1 Introduction The pinch based nuclear fusion experiments produce

fast (2.5/ 14 Me V) neutn~.!1s 1~6 in a burst (duration 10-9 to 10-7

, intensity 105 to 1012 neutrons). The electrical driv­

ers, used for these experi ments, produce strong electro­magnetic pu lsed rad iations, ranging from few tens of

kHz to radio- frequency (RF). The pinches also produce

radiations from microwaves to hard X- rays. Therefore

the resultant electromagnetic pul se (EMP), accompany­

ing the neutron burst, has spectrum extending from few tens of kHz to hard X-rays (till MeV) . This EMP can,

not only interfere in the neutron measurements, but can even damage the measuring equi pment, unl ess adequate precautions are taken.

We have developed/modified several neutrons detec­

tion techniq ues, so that they work re li ably in a strong EMP environ ment. These techniques are ( I) Off line

techniques such as acti vati on, so lid state nuclear track,

bubble detectors etc. (2) Sc in t illator detectors (3) Neu­

tron Pul ses Stretching Tech niques. These techniques are discussed w;th a specia l emphasis on thei r protection from EMP.

2 Experimental Techniques Activation detection - Act ivati on technique7 is the

1110st re li able and anc ient method of neutron detection.

However the tech nique (or more exactly the activation materia ls) is mod ified to detect neu trons emitted in a

burst. The activati on A of the sample is given by : A::::: G.a . d .~exp(- /dp) A = 0.69/TI1 .. . ( I )

· Current ;\dd~ess: Insti tute: for Plasma Research. Hha!. Gandhinagar. Gujarat 382 428

where G, a, d are activat ion cross-section, area and thickness of the foi l respect ively. <j> is the neutron flu::, tp is the activation time and Til is decay half li fe of the .. d 8 activation pro uct . In burst neutron experi ments the pulse durallc ')

tp < Til . Therefore from Eq.( I) : A ::::: G .a.d .<j>tp/TI1 ( } From Eq. (2), it can be inferred, that unli ke fi ss ion

reactor (or any other steady neutron source), the halflife of the resultant e lement, after activat ion, should be small. Therefore acti vati on materia ls, such as gold . which are suitabl e fo r reactor studies, are not used fo r burst neutron sources.

I f the energy of the neutrons is few Me V. than better sensitivi ty is obtained, if the neutrons are first thermai­ised in an hydrogenous materia l. The best [ as ~ er

Eq. (2)] e lements fo r therm al neutron act ivation are rhodium , silver and indium (see Appendi x8

.9

). Rhodium is margina lly superi or to sil ver, but is very ex p iLi\ c. therefore si lver is the activat ion mater ial norm all) l,s.:d (comm ercially ava il ab le silver .999 is adeq uate :

The silver foi l th ickness was opt imized for 13 X;I' .l,;-'

neutron absorption s and mini mu lll r3 attention I el;~ '~(~d

by activated nucle i - see Appendi x I). A foil t hl cl\ n es~

250 flm g ives the best results. Polythene was u ed a ~·

thenna li sing material (any other hydrogenous m"tertai can al so be used). 0.02 m ofpo lythene was kept be~\\ een source and fo i I (as in fe rred by our compute r s i mub ti\~ 'l:'.

the neutrons were slight ly under thermali ed lC' ~.lM:

advantage of g iant resonances of activation cross<ec­tion at epitherm al neutron energies) . Higher is the th IC!-; ­

ness of polythene on a ll other sides of foil. the better i ~

detecti on thresho ld, norm ally thickness of 0.2 5 m IS

450 INDIAN J PURE APPL PHYS. VOL 37, JUNE 1999

adequate. Even lower detection threshold is obtained if the source is also surrounded by hydrogenous material.

High energy neutrons ( =14 MeV), can be more efficiently detected by direct activation (i.e. without any thermal ising). The best element for this is lead (see Appendix I). Typically a plate of I 0-' m thickness is used for activation .

In our detecting system, we used O. I m diameter si lver foil. After activation the foil activity was counted using a 13 detector. It consisted of a plastic scintillator (Be 400, 0.1 m diameter and 0.0 I m thick) mounted on 0.1 m diameter photo multiplier (PM) tube (EMI 9530). The output of the PM tube was amplified using a pream­plifier and amplifier and then counted using a multi-sca­lar connected to a personal computer for data storage. The activation foi ls were kept close to the experiment, to get better sens itivity. The detecting electronics was housed in side an electromagnetically shielded enclo­sure2 to prevent the detection electronics from malfunc­tioning or damage due to EMP. These precautions were adequate for reliable detection of pulsed neutrons. Our well optim ized systems have a detection threshold of 5.104

, eutrons. Fig.1 shows the schematic of the system. Fig. '2 shows the counts observed after the foil was activated by a neutron (energy 2.5 MeV) burst produced a pbsma focus pinch device' o.

olid Slale Nuclear Track Detectors - Solid state 7 nuclear track detectors (SSNTD) record the damage (as

tracks) by the energetic charged particles. The neutrons can be converted to charged part ic le by proton recoil (see Appendix I) and the protons can be detected by SSNTD such as CR 39. They can also be converted to charged particles using sui table nuclear reactions with 8 10, Li6 or U235 etc (see Appendix I). These charged particles can be recorded by SSNTD such as LR 115 . We have tested both the techniques and found that they are an order of magnitude less sensi ti ve as compared to activation detectors. The processing and counting of tracks is also very cumbersome.

"The shie lded enclosures (size 4.8 m x 2.4 m x 2.4 m) used were professiona lly engi neered state of art systems (obtained from RFI, Austral ia and Siemens, German ). The systems could provide > 100 d8 attenuation, both fo r translll itted as well as conducted electromagnetic interference > I GHz. As an added precaution the enclo­sures were provided a separate and exclus ive ground . The power to enclosure was provided through iso lation transformers.

Bubble Detectors - Bubble detectors" are miniatur­ized bubble chambers', These detectors contain super­heated liquid (such as freon) in suitable matrix (it is a gel of hydrogenous material for detection of fast neu­trons by proton recoil- see Appendix I) , When a charge particle (produced by neutrons) interacts with the ma­trix, the super heated liquid vapourises and forms a bubble.

The best threshold, experimentally determined, for a single detector is 5.105 neutrons. A lower threshold, can obviously be obtained, by using several detectors. Bub­ble detectors are the most convenient detectors and are very insensitive to electromagnetic radiations . However these have a short life of few months and bubbles can also be formed by heat.

Scintillation Detectors - Scintillation detectors7 can be used for measuring temporally resolved neutron emission and the spectrum of neutrons4

.12 . The tech­

niques used for detecting individual neutrons, such as pulse shape discrim ination 7, cannot norma lly be used for burst emission as the scintillators detect the burst in integrated mode .

For fast neutrons plastic scintillators are preferred as these detectors have lowest (= 2ns) decay time. The scinti lIators detect neutron burst in integrated mode, by proton recoil (see Appendix I). To make a fast and efficient detector a large diameter, fast PM tube is used (in our experiments Philips XP 2041, 150 mm diameter, = 2 ns rise time was employed). For good effic iency (= 40 %) the plastic scintillators (Be 400) of 0.05 and 0.15 thickness are employed for 2.5 and 14 Me'l neutrons respectively.

Plastic scintillators are also very sensi tive to Xlyrays. The only reliable way to discriminate neutron burst from these is to use time off1ight techn ique If the pulse wid th of the emission (neutrons and y) is t then , for ylX-ray discrimination the minimum distance (1\) of the detec­tor, from source is given by Eq. (3)

T=Nv-i\/c .. . (3)

where v and c are the ve locities of neutrons and electro­magnetic radiations respecti vel y (3 .1 OS m/s). Since

v=(2 E/m) I/2 .. . (4 )

\\ here E and 111 are neutrons ' energy and mass respec­tive Iy. Therefore from Eqs (3) and (4) the energy spectra of the neu trons can also be determined. However if the initial pul se shape of the burst is not known than t'N O

detectors are required, positioned at two diffe rent dis­tances. I fthe spectrum of th e neutrons also changes with time than seve ral detectors, positioned at different dis-

SHY AM: NEUTRON DETECTION IN EM PULSE 451

tances , will have to be employed2.6 (in one of our

experiments six detectors were used). And neutron spec­

trum will have to be unfolded . The accuracy of the

unfolded spectra will depend on the number of detectors

used . To prevent detectors from getting affected by RF

noise, they were enclosed in a 0 .003 m thick mild-steel

housing. The high voltage as well as signal cables were

coaxia l cables enclosed in copper bellows. The high

vo ltage power-supply as we ll as storage oscilloscope (to

PULSED NEUTRON SOUHCE

TRERlULlSlNG HYDROGENOUS IUTEm,u.

record signal) were in the electromagnetically shielded enclosure .

To further prevent the electromagnetic interference from affecting system the preamplifier was di spensed with altogether. To achieve this, a high current PM tube (Philips XP 2041 -linear till 300 mA anode current) was used . It was operated with negative high voltage so that the anode pulse base was at ground potential. The load resistance was of 50 ohms (equal to the characteristic impedance of cable). The dyanode chain current was relatively high (= 2 mA) and capacitors (= 2 nF) were

:.J~ ~-~;'~~r ~! ::I.T!PI.IEP. t; '~ IIIGK VOLTAGt:

~! ·;JCru

C~~ AMPLIt·!P.R WULT! SCALAR

ELECTROIUG~'F.TICALLY SHIEUlED [!'CLOSURE

Fig. I - Schematic of activation detector system. The setup shown is for silver activation . The silver foil (250 11m thick. 0.1 m diameter) is surrounded by polyethylene on all sides for neutron thermalisation . The hurst neutrons were produced by a plasma focu s fusion device. The silver. after irradiation. is removed from the thermalisation assembly and plact'd on the P detector (plastic scint illator mounted un photo-multiplier tuhe) kept in the electromagnetically shielded enclosure

(i) '0 c ro If)

::J 0 ..c ~ '-'

ro ~ <l.l C L-

<l.l 0.. If)

c ::J 0 0

Silver Activation Detector Counts

as a Function of Time

8 jf-' .~ :::-- .

1--Counts, Expected 1 _ + Counts, Observed

.~

\

4 , ~

~ \

2 \ ~

'--I; ~ - .

O · ---'--~

0 50 100 150 200

Time,

Fr I' . 2 - The count from activated silver when placed on the P detector. The counting interval was 4 s. The line and the so lid circles arc \IlL counb expected fro :;} exponential d.::cay of activity. The + mark s arc the actual counts observed

452 INDIAN J PURE APPL HYS. VOL 37, JUNE 1999

connected in parallel to each of the dyanode chain resis­tors.

is recorded on a high bandwidth (> 100 11HZ) storage oscilloscope. Fig. 4 shows the time of fl ight signal. The first pulse is due to hard X-rays and second pulse is of neutrons. The radiations detected were ,emitted from a low energy plasma focus device and the neutron energy was approximately 2.5 MeV.

Attempt was also made, to use optical fibres, to bring scintillator optical signals, directly, to the shielded en­closure (to reduce effect of EMP). However, it was discovered, that optical fibre cannot be used in an envi­ronment 'having very high y/X-ray background, as the fibres themselves scintillate and these parasitic lumines­cence drowned the genuine signal.

Neutron Burst Stretching Techniques - In this tech­nique, the neutron burst is allowed to thermal ise in appropriately sized hydrogenous material (typical di­mension 0.25 to I m - see Fig. 5). Because of the ~tatistica l nature of neutron thermal is ing process, the

-- Fig. 3 shows the schematic of the time of flight ':!etector setu~ . The pulse from the photo-multipl ier tube

V\;LSEO NEUTRON SOURCE

rHOTOllULTIPUER rnl STORAGE

TUBE +-~J OSCIlLOSCOPE

~--L-----'t-.~j _J ~LTAGE ~ I . -~-R\;~l:PPLY SCINTIL.UTOR PLASTIC

Fig. 3 - The tim e of flight setup. T he detector, consisting of pl astic scinti llator and photo multip llt:r tube. is 6 m a vay from a ;> lasma

focus [l nch neutron source. The s ignal is recorded on a high band width storage oscil loscope. housed in an e lecrromagnetica!l shielded en­dc-surt:

O ~---'-(r---------~------___ ~

~~ . V ~ -60 -

-80 ~

~rime (ns)

Fig. 4 - Signal from lime of !l igh t · etector reco rded on L oscilloscope (time base 0 ns/small di vis ion). The firs t pulse i.; due to hard X-rays and the second pulse rec rJ~ the n lI tron bursl

I -~ -ttlo I ~~~it~~coPr - I ' --- I L

1[>= I r-----' • 1311 I I 2 ___ ----t-1 - ~!Iffi!vbl.TAGE 1_ L .. , : L ---- POWER S~rrlY

L _-.-l ; 1 UllUUM GUSS SCINTIu.ATOn 2 PDOTO U.ULTrPUER rum; 3 llYDROCENOUS ldATERl.AL 4 PULS£V Sr.l.TiRON soCHeE 'I £LEC1HOMll''; :-;L'':'1CALLY

_ _ _ _ S1.~~~ .. ~~:!OSIlN.-E---'

F·g. 5 - Th.: schematic of'neutron burst stre tch in.g (echlllq ue . T he n.:utrons burst IS thermaiised and stretched to fe v tens o(

m icroseconds. The thermalised neutrons rc dctect.:d by ilhlll ll1 glass sCl ct illato r l1lollntcd on a phmo m ulti!J! ie luhe The Oll irut is recorded 0 11 an oscilloscope and the putses are counted

SHYAM: NEUTRON DETECTION IN EM PULSE 453

10 ~----------------------------~

5

-5

-10

-15~--~-------~--~----------~-------~--~ o 100 200 300 400 500 600 Time (micro-second)

Fig. 6 - The siglIn I from the lith ium g lass detector recorded on the osci lloscope. The fi rst pulse is due to X-rays and radio-frequency no ise: the subsequent pul ses an: due to indi vi dual thermalised neutrons

thermalised neutron burst stretches to few tens of IlS .

The thermalised neutrons can be detected and counted . . d ' t11 4 usmg appropnate etectmg system .. . An array of boron tri - fluoride or helium-3 fill ed pro­

portional detectors can be lIsed for detecti ng thermal

neutrons. However since these a re high impedance de­

tectors and they are not sui table in high EMP environ­

ment. A better detector is lithium-6 glass sc intillato r. We

used 0 .003 m th ick glass scinti llator (I E 905) mo unted

on a 0. 1 m diameter PM tube (EM I 96 18). T he system

was embedded in 0 .3 m diameter perspex block. The size

was optimised using monte-carlo neutro nic sim ulation.

The neutron burst was stretched to 50 flS. T he output of

the PM tube was recorded on. a sto rage osc i lIoscope. T he

threshold of the system was ~ 50 neutrons . Fig 6 shows the stretched neut ron bursts recorded on

a stor?ge oscilloscope. The first pu se is due to EMP and

tne subsequent pulses are due to s lI1g le therma lised

neutrons .

3 Discussion We have iscussed several techniques to rel iabl y

etect neutron burst emi tted a ong with strong EMP.

There are other techniques, ,vhich may also be used . but

we have confined ourselves to technio es wh ich ve had

used and evaluated in our laborato ry .

Acknowledgements I am grateful to Dr H Schmidt and Dr R Schn id t,

lnsti tut Fur Pia mafo rch ung, Stuttoart, FR German), and

Dr RK R Oll , Dr TC Kaushi k and Sh -i L Kl lb.rni,

Bhabha Atom ic Research Centr, umbai.

A ppendix I It is well known that neutrons (n) are detected only

via production of other (charged) particles. These parti­cles can be created , ma inly by three methods

(1) Activat ion Reactions

Making a su itable (activation) material radioactive a by absorpt ion of neutrons and measuring the radioactiv­i y. Some of the use ful activat ion react ions, having half lives < I minute, are

IOJ Rh +n ~ I04 Rh ~ I04 pd + ~(2.44 MeV) ... (i)

sotopic abundance = 100% alii = 140 b Tt l2=44 .0s

109 Ag + n ~ 110 .g ~ II Oed + ~(2 . 87 MeV) .. . ( ii)

Isotop ic abundance = 49% a lll = 113 b T l/2 = 24.5 s

11 5!n + n ~ 1161n ~ 116Sn + ~(3 . 29 MeV) .. . (i' i)

Isotopic abundance = 96 % a l ii = 52 b TI I2 = 1 .4 s

20RPb + n - r 20711l Pb +2n ~207Pb + y( 1.06 MeV) .. (iv)

Isotopic abundance = 28% 0'1 4 = 1.3 b TI 12 = 0.797 <;

where Ag, h and Pb are silver. rhodium and leaa r~specti vely . The superscr ibed nu mber on their left are heir atom ic masses . a lii anJ 0' 14 are neutron acti vat ion crc~S- . ect ions a t thermal (0 .025 eV ) and 14 MeV e­~')ecti v ciy Tt.2 is the hal f-I i fe of the acti 'a ted nuclei

(2) Prompt Nuclea r React ions

Disintegrati ng an appropri ate isotope by neutron ab­sorpt ion and measuring the charged frag ments prod uced

ill _ .L n -4 7Li .!. a + 4 .8 \1eV .. (v)

454 INDIAN J PURE APPL PHYS. VOL 37, JUNE 1999

Isotopic abundance = 20 % crtl. = 40 lOb

6Li + n ~ t + a + 4.8 MeV ... (vi) Isotopic abundance = 7.5 % crtlt = 940 b

3He + n ~ t + p + 0.76 MeV ... (vii) Isotopic abundance = -- crllt = 5330 b

235 U + n ~ Fission fragments + 200 MeV ... (viii) Isotopic abundance = 0.72 % crllt = 540 b

(3) Proton Recoil Making a neutron collide with a charged (normally

with protons in a hydrogenous material), a part of the' energy gets transferred to charged particle and the resul­tant energetic charged particle is detected. To a proton, a neutron, on collision, will transfer on an average 50 % of its energy. The spectrum of the recoil protons is uniform from 0 to the neutron energy.

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